1. Field of the Invention
The present invention relates to a process for making an oxide dispersion-strengthened tungsten heavy alloy mainly used for an amour-plate-destroying warhead by mechanical alloying.
2. Description of the Related Art
Tungsten heavy alloy is mainly comprised of 90 to 98 wt. % of tungsten to use the high-density property of the tungsten, and nickel and iron for the rest at a weight ratio of 7:3 to 8:2 to enhance the sintering property and processability. Such a tungsten heavy alloy has a fine structure in which body-centered cubic (BCC) spherical tungsten grains with a particle size of 30 to 40 mm are dispersed in the face-centered cubic (FCC) Wxe2x80x94Nixe2x80x94Fe matrix. The tungsten heavy alloy possesses excellent mechanical properties, for example, high density of 16 to 18.5 g/cm2, high tensile strength of 800 to 1200 MPa and elongation percentage of 20 to 30% and is broadly used as a material for the balanced supports of aircrafts, radiation shielding devices, vibration attenuators, and the piercers of amour-plate-destroying kinetic energy piece. The conventional tungsten heavy alloy is manufactured by a powder metallurgy process that involves sintering mixed powders of tungsten, nickel and iron at a temperature above 1460xc2x0 C.
The amour-plate-destroying piercers using a high kinetic energy to pierce the amour plate are made from depleted uranium or a tungsten heavy alloy that has high density, high strength and high impact energy. The amour-plate-destroying piercer made from depleted uranium is excellent in the piercing ability but currently limited in its use because it is disadvantageously poor in processability and abrasion resistance and, especially, incurs radioactive contamination. The amour-plate-destroying piercer made from a tungsten heavy alloy has a smaller piercing depth per density than the amour-plate-destroying piercer made from depleted uranium in piercing the amour plate. For that reason, many studied have been made on a material for the amour-plate-destroying piercer replacing the depleted uranium with improved mechanical properties of the tungsten heavy alloy.
The oxide dispersion strengthening method involves dispersion of a thermodynamically stable fine oxide in a metallic matrix without reaction with the base phase to fabricate a high-temperature material, which is advantageously excellent in high-temperature mechanical properties and usable at a high temperature approximately up to the melting point.
The known oxide dispersion-strengthened alloys are as follows. The first commercially available oxide dispersion-strengthened alloy was ThO2 dispersion-strengthened tungsten (GE, U.S.A., 1910) and developed as TD-Nickel (Du Pont, U.S.A., 1962) containing 2 vol % of ThO2 dispersed in the Ni matrix as a structural material. The earnest applications of the oxide dispersion-strengthened alloys started from the manufacture of Y2O3 dispersion-strengthened Ni-based superalloy using the mechanical alloying technique by Benjamin at Inco Co. (U.S.A) in 1970. The oxide dispersion-strengthening technique has been applied in the manufacture of dispersion-strengthened Cu alloy, dispersion-strengthened Al alloy and dispersion-strengthened W alloy as well as Ni-based superalloy, which are used as a material for aircraft engine, welding electrodes, electrical contact material, and so forth.
No document has been found that suggests the process for making an oxide dispersion-strengthened tungsten heavy alloy by mechanical alloying and the use of the oxide dispersion-strengthened tungsten heavy alloy for amour-plate-destroying piercers. Especially, it is preferable for the amour-plate-destroying piercers to be susceptible to local fracture with concentrated stress under the shearing strain around the oxide dispersed in the matrix in order to have an enhanced piercing performance, since the piercers need the self-sharpening effect by which the edge of the piercers is fractured to make the piercers sharpened. However, there is still no report on the systematic research on the process for making the oxide dispersion-strengthened tungsten heavy metal, in particular, enhancing the self-sharpening by the oxide dispersion.
Accordingly, the inventors have studied on the subject and found out that an oxide dispersion-strengthened tungsten heavy alloy containing a uniform Y2O3 dispersion in the tungsten heavy alloy can be manufactured from a composition including Y2O3 powders and tungsten heavy alloy powders by mechanical alloying and liquid-phase sintering.
It is, therefore, an object of the present invention to provide a process for making an oxide dispersion-strengthened tungsten heavy alloy with enhanced high-temperature compression strength and susceptible to fracture during shear deformation by dispersing an oxide in the matrix uniformly and minutely by a mechanical alloying.
To achieve the above object of the present invention, there is provided a process for making an oxide dispersion-strengthened tungsten heavy alloy by mechanical alloying, the process including the steps of: adding 0.1 to 5 wt. % of Y2O3 powders to mixed powders comprising more than 90 wt. % of tungsten powders, and nickel and iron powders for the rest; subjecting the resulting mixture to a mechanical alloying to prepare oxide dispersion-strengthened tungsten heavy alloy powders; compacting the tungsten heavy alloy powders into compressed powders using a press; and sintering the compressed powder at a temperature in the range of 1400 to 1600xc2x0 C.
The oxide dispersion-strengthened tungsten heavy alloy prepared by the mechanical alloying is characterized in that the fine Y2O3 particles are uniformly dispersed in the matrix. The addition of the Y2O3 particles stable at high temperatures results in enhanced high-temperature strength and a reduction of the shearing strain of the fracture during high-speed shear deformation, based on which fact, a technique has been developed to control the mechanical properties of the oxide dispersion-strengthened tungsten heavy alloy by varying the added amount of the Y2O3.
The tungsten heavy alloy uses the high density (19.3 g/cm3) of tungsten and its chief application, e.g., the amour-plate-destroying kinetic energy piercer has the piercing performance enhanced with an increase in the density. The depleted uranium alloy used for the kinetic energy piercer has a high density, for example, 17.2 g/cm3 for DU-8Mo and 17.3 g/cm3 for DU-6Nb. For that reason, the tungsten heavy alloy suitable for the kinetic energy piercer must have a high density of more than 17 g/cm3 so that it has to contain more than 90 wt. % of tungsten.
The conventional ball milling method using low energy simply mixes the powders without a reduction of the grain size or the lamella distance. The most important thing in the process for making the oxide dispersion-strengthened tungsten heavy alloy is uniformly dispersing the fine oxide particles in the matrix. Thus the present invention utilizes the mechanical alloying method to obtain tungsten heavy alloy powders having a reduced grain size of less than 100 nm and a reduced lamella distance of less than 0.5 xcexcm.
The added amount of the Y2O3 is preferably in the range from 0.1 to 5 wt. % based on the total weight of the tungsten heavy alloy. The amount of the Y2O3 less than 0.1 wt. % hardly contributes to the reduction of the tungsten grain size or an increase in the high-temperature mechanical properties, whereas the amount of the Y2O3 more than 5 wt. % rather results in a deterioration of the mechanical properties and mechanical processability.
The tungsten alloy powders obtained by the mechanical alloying method of the present invention are then compacted under a pressure into compressed powders and then subjected to a sintering in the temperature range of 1400 to 1600xc2x0 C. The sintering temperature below 1400xc2x0 C. hardly achieves sufficient sintering and reduces the relative density of the final product with a deterioration of the mechanical properties, whereas the sintering temperature above 1600xc2x0 C. excessively increases the liquid phase fraction and the diffusion velocity to result in a rapid growth of the tungsten grains with a deterioration of the mechanical properties.
The sintering step is preferably performed under the reducing atmosphere, for example, hydrogen atmosphere in order to reduce the oxide that is contained in the metal powders and inhibits the densification. As such, a second heat treatment is preferably performed under the nitrogen atmosphere so as to remove the residual hydrogen.